Double cryostat



July 2, 1963 H. P. WURTZ, JR 3,095,711

DOUBLE CRYOSTAT Filed Jan. 51, 1962 INVENTOR. HOWARD P WURTZ,Jr.

gas takes place.

United States Patent 3,095,711 DOUBLE CRYOSTAT Howard P. Wurtz, Jr., Santa Barbara, Calif., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Jan. 31, 1962, Ser. No. 170,293 5 Claims. (Cl. 62-514) This invention relates to gas expansion cooling devices, and more particularly to Joule-Thomson cryostats.

Joule-Thomson cryostats were developed to provide a miniature cooling device for attaining the low temperature necessary for certain infrared wavelength detectors. Essentially they consist of a short length of thin metal tubing of small size wound about a mandrel. This unit is installed in a close-fitting tube comprising the inner portion of a small vacuum flask. High pressure gas is then directed through the tubing towards the bottom of the vacuum flask where it is released through a pressure drop or expansion nozzle. The gasreleased from the nozzle travels back up the flaskaround the tubing between the inner walls of the vacuum flask and the outer surface of the mandrel, wherein further expansion of the cooling As the gas expands, it is cooled by Joule-Thomson effect of expansion, and as it is directed back over the coils of the tubing, a regenerative cooling effect is produced. When pressurized nitrogen gas is used, as is most common, the gas is cooled enough to liquify at a temperature close to -l96 C.

When the operation of the Joule-Thomson cryostat is initiated, the amount of time required to reduce the temperature from room temperature to the very low temperatures desired will be determined by the flow rate of the gas passing though the tubing. After the operating temperature has been attained, the rate of flow of the gas to maintain this desired temperature is somewhat lower than that required to reach it. To avoid undue standby delays, the initial flow rate should be substantially larger than that required to maintain the operating temperature. The continuation of the substantially larger flow rate after the cryostat has reached operating condi tion, however, will result in substantial waste of the high pressure gas. Assuming a constant gas pressure at the inlet to the tubing, lower flow rates (higher gas economy) must be gained at the expense of longer starting time. The reverse, of course is also true. Previous attempts to solve this problem have included such devices as adjustable orifices, whichwere adjusted after reaching the operating temperature to reduce the flow of gas to the lower rate. Their function was to provide a large flow rate for quick starting and then reduce the flow during operation to a more economical rate. However, certain difficulties in the manufacture and maintenance were presented in the use of these devices due to the fact that extremely small valves and openings are required by the miniature cryostats, and also these valves are required to function accurately at or near the temperature of the liquified gas. Such precision valves and throttles of such miniature size were expensive to produce and required the use of highly purified gas to prevent their fouling by impurities.

An object of this invention is to provide a Joule-Thomson cryostat which has the advantages of gas economy and fast starting and is also inexpensive and easily constructed.

Various other objects and advantages will be apparent from the following description of one embodiment of the invention, and the novel features will be particularly pointed out hereinafter in connection with the appended claims.

In the accompanying drawings:

FIG. 1 is a schematic drawing of a double helix cryo- 3,095,711 Patented July 2, 1963 stat arrangement constructed in accordance with this invention;

FIG. 2 illustrates a schematic diagram of a coaxial double cryostat according to the invention.

In FIG. 1, a Dewar flask is shown placed in an upright position. The Dewar flask 10 has inside walls 11 and outside walls 12 defining an evacuated space 13 therebetween. The walls of the flask are constructed of a non-heat conducting material to prevent the passage of heat therein between the inside of the flask and the higher temperatures of the outside portion of the flask. The center of the Dewar flask provides a cylindrical shaped envelope into which a mandrel 14 is inserted. A small nipple 15 located on the end of the mandrel 14 is provided to maintain the main body of the mandrel slightly above the bottom of the flask thus giving a small open space at the bottom thereof. A pair of finned tubes 16 and 17 are wrapped in parallel helical fashion about the mandrel 14. The helical coils of this tubing fit tightly between the mandrel and the inner walls of the Dewar flask. At the bottom of the flask, each of the finned tubes 16 and 17 has an expansion nozzle or throttling opening 18 and 19 respectively, which extend into the small open space below the main body of the mandrel created by the nipple 15. The finned portions of the tubing 16 and 17 are connected by regular tubing through two separate paths 21 and 22 respectively to a source 23 of high pressure gas, such .as nitrogen. The high pressure source 23 has -a gas inlet fitting 24 on which is located a valve 25 for emitting the high pressured gas to the system. A cutoff valve 26 is located in the path 22 in such a manner that when this valve is closed the gas flow through that path is stopped; this valve may take any well-known form which will provide the necessary open and close operation for the path 22.

, tubing 16 and 17. The high pressure gas travels through both tubes in a helical path and is discharged through the nozzles 18 and 19 at the bottom of the flask. The discharged gas expands at the nozzle openings and provides cooling in accordance with the theory of the Joule-Thomson effect. The released gas passes back up over the outside of the finned tubes thereby cooling the tubing and the gas within due to the heat conductive properties of the tubing material. The regenerative cooling effect produced by the expanded gases continually cools the high pressure gas within the tubing, which when later discharged isrfurther cooled by the expansion.

As before mentioned, the time required for the cryostat to attain a given operating temperature will depend upon the flow rate of the high pressure gas in the tubing. The expansion valve opening 18 at the end of the finned tube portion 16 is constructed to admit only a suflicient amount of high pressure gas to maintain the cryostat at the desired operating temperature. The expansion nozzle 19, however, may be constructed to admit a much larger flow of gas.

When the desired operating temperature has been reached the cutoff valve 26 is closed to thereby stop the flow of gas in the path 22. The closing of the cutoff valve may either be accomplished manually or by some sort of thermostat arrangement which senses the desired temperature within the cryostat and automatically closes the valve 26; the method by which the valve 26 is closed, however, is not the subject of this invention. Thus after the operating temperature has been reached, the flow rate is cut down to an amount which is just sufiicient to maintain the desired temperature. The additional flow through the path 22 'allows the cryostat to reach a given operating temperature in a relatively short time; then by cutting off this added flow waste of the gas after the cryostat has reached the desired temperature is prevented.

The coaxial double cryostat of PEG. 2 employs much the same principle, but actually consists of apair of 'cryostats, one for running and one for starting, placed one within the other with a heat exchange path therebetween. The finned tubing 31 for the running cryostat is wound about the hollow mandrel 32 and has its expansion valve opening 33 located at the bottom portion of the Dewar flask 29. The mandrel in this case, is hollow and has heat conducting properties thus to provide a heat conductive path between the outside cryostat and that located within the hollow mandrel 32. The second portion of finned tubing 34 is Wound about an inside mandrel 35 located within the hollow mandrel 32 with its expansion valve opening 36 located at the closed end of the hollow mandrel. Both portions of finned tubing 31 and 34 are connected, as before, by conventional tube paths 37 and 38 to a source of high pressure gas 39 through a gas inlet fitting 41. A cutoff valve 42 is located in the gas path 37 to effectuate the stoppage of gas flow through the finned tube 34 located on the inside of the hollow mandrel 32 at the desired time, when the double cryostat has reached operating temperature.

Upon starting the coaxial double cryostat, the inlet valve 43 on the gas inlet fitting 41 is opened to admit high pressure gas flow from the high pressure gas source 39 through each of the paths 37 and 38 to the finned tubes 34 and 31, respectively. Initially the cutoff valve 42 is placed in its starting or opened condition to permit the high pressure gas flow through the path 37 to the starting cryostat located within the hollow mandrel 32. Thehigh pressure gas released from the inside expansion valve opening 36 expands in the bottom portion of the hollow mandrel and flows up over the thin tubing to provide the aforementioned regenerative cooling effect. The same regenerative cooling effect is taking place also in the outside or running cryostat by the release of the high pressure gas from the expansion gas opening 33. The heat conductive walls of the hollow mandrel 32 provide a heat exchange path between the inside and the outside cryostat thereby giving a double cooling effect within the Dewar flask 29 as long as the valve 42 remains open. As before mentioned, the gas flow through the thin tubing 31 on the outside of the running cryostat is made just sutlieient to insure that a desired operating temperature can be maintained once such temperature has been reached by the cryostat. The added fiow within the inside or starting cryostat may be made suificiently large to obtain the desired quick cooling in reaching the desired temperature. When that temperature has been reached, the shut off valve 42 is closed automatically or manually to-stop the flow in the inside cryostat thus leaving the outside cryostat with its smaller flow to maintain the temperature of operation. It will be obvious to any person skilled in the art that the roles of the inside and the outside cryostat may be reversed Without substantially afiecting the operation of the coaxial double cryostat.

The fast starting times coupled with gas economy by the simple operation of a cutoff valve makes the use of the double cryostat especially applicable where weight considerations and simplicity of operating components are required, such as in contain infrared detecting operations performed in aircraft and missiles.

It will be understood that various changes in the details, materials, and arrangement of parts, which have been hereindescribed and illustrated in order to explain the nature of the invention, may be made by those skilled in the art, within the principle and scope of the invention as expressed in the appended claims.

What is claimed is:

1. A Joule-Thomson effect cryostat comprising a hollow envelope closed at one end, a hollow mandrel disposed at the center of said hollow envelope, a first hollow, heat exchanging tube spirally wound within said hollow mandrel, a second hollow heat exchanging tube wound on said mandrel, each of said hollow, heat exchanging tubes having an expansion nozzle opening at the closed end of said envelope, means for introducing a high pressure gas at the other end of said tubes for flow therethrough, and valve means for closing one of said tubes to gas flow therethrough.

2. A Joule-Thomson effect cryostat comprising a hollow envelope closed at one end, a hollow mandrel disposed at the center of said hollow envelope, a pair of hollow heat exchanging tubes disposed within said envelope, one of said tubes being disposed within said hollow mandrel and the other of said tubes being wound on said mandrel, each of said tubes having a gas discharging opening at the closed end of said hollow envelope, means for introducing high pressure gas at the other end of said tubes for flow therethruogh, and valve means for closing one of said tubes to gas flow therethrough.

3. The Joule-Thomson eflect cryostat of claim 2 wherein said hollow mandrel is closed at one end, said closed end of said mandrel being located at the closed end of said hollow envelope.

4. The Joule-Thomson effect cryostate of claim 3 wherein said valve means closes said one of said tubes wound within said hollow mandrel.

5. A Joule-Thomson efiect cryostat comprising a first hollow envelope closed at one end, a second hollow envelpe disposed coaxially within said first hollow envelope, said second envelope being closed at one end and consisting of a heat exchanging material, a first hollow, heat exchanging tube disposed on said second envelope, a second hollow, heat exchanging tube disposed within said second envelope, both of said tubes having a gas discharging opening at the closed ends of said hollow envelopes, means for introducing high pressure gas at the other end of each of said tubes for flow therethrough, and valve means for closing one of said tubes to gas flow there through.

References Cited in the file of this patent UNITED STATES PATENTS 2,991,633 Simon July 11, 1961 

2. A JOULE-THOMSON EFFECT CRYOSTAT COMPRISING A HOLLOW ENVELOPE CLOSED AT ONE END, A HOLLOW MANDREL DISPOSED AT THE CENTER OF SAID HOLLOW ENVELOPE, A PAIR OF HOLLOW HEAT EXCHANGING TUBES DISPOSED WITHIN SAID ENVELOPE, ONE OF SAID TUBES BEING DISPOSED WITHIN SAID HOLLOW MANDREL AND THE OTHER OF SAID TUBES BEING WOUND ON SAID MANDREL, EACH OF SAID TUBES HAVING A GAS DISCHARGING OPENING AT THE CLOSED END OF SAID HOLLOW ENVELOPE, MEANS FOR INTRODUCING HIGH PRESSURE GAS AT THE OTHER END OF SAID TUBES FOR FLOW THERETHROUGH AND VALVE MEANS FOR CLOSING ONE OF SAID TUBES TO GAS FLOW THERETHROUGH. 